'A GREAT sense of presence and elegance and simplicity...." is Prince Charles's verdict.He made his favorable pronouncement (for the TV cameras) in pointed but quiet phrase inside what the London Times in its turn pronounced "the finest new art gallery in Europe." This somewhat over-the-top accolade was given to the recently opened extension to London's National Gallery, the "Sainsbury Wing," on Trafalgar Square, designed by postmodern American architect and theoretician, Robert Venturi. The long-awaited building is on a corner site, and houses the gallery's rearranged Early Renaissance paintings, both Italian and Northern. It gives this remarkable collection, much of it originally painted for architectural settings (though some pictures are small and intimate) a context of cool, classical probity. Some visitors like the prince find "the interior spaces" a "triumph." Others find them so uninteresting and mute as architecture that the galleries offer no distraction at all from the (superbly well-lit) paintings. That might in the end be the highest praise for Mr. Venturi's design. Apart from a monumental staircase that is in some people's view quite unnecessarily grandiose, his building is positively monastic in its self-effacement and hard-surfaced plainness: and this from an architect who once countered modern architect Mies van der Rohe's dictum "less is more" with "less is a bore." Architect Maxwell Hutchinson (who until July this year was president of the Royal Insitute of British Architects), in his 1989 book "The Prince of Wales: Right or Wrong?" calls Venturi's building "a study in invisible architecture." And he adds: "It is skillful, deferential and anodyne. It is, in other words, exactly what the prince wants." "What the prince wants" has become an extraordinary and contentious side-issue to the world of British architecture in recent years. The National Gallery Extension (as well as the argued-over Paternoster Square site near his loved St. Paul's) has been symbolic of the prince's highly opinionated interventions designed to upset the "arrogance" of professional architects. The prince speaks (he believes) on behalf of the hitherto silent majority of ordinary-people-who-can't-stand-modern-architecture - particularly 1960s tower-blocks, "glass stumps," and such "dreadful" buildings as Mondial House by the Thames, an "excrescence" that is "redolent of a word processor." But it was in 1984 that Prince Charles made his most quoted remark about despised modernist architecture. In a speech at Hampton Court, where Tudor royal taste in architecture is opulently displayed, he said that what was then proposed as the National Gallery's Extension was "like a monstrous carbuncle on the face of a much-loved and elegant friend.... Why," he asked, "can't we have those curves and arches that express feeling in design? What is wrong with them? Why has everything got to be vertical, str aight, unbending, only at right angles - and functional?" So the trustees got cold feet about their proposal. A firm of architects lost a job. (Not the last to actually lose important business as the result of the prince's often extremely personal attacks.) And - seven years later - here is Venturi's concept, with, it must be admitted, lots of right angles, but also a great many arches. Talking about the extension's exterior - which plays a clever game of harmonizing with the older gallery itself and its classical columns - the prince effectively closed the chapter he had started at Hampton Court. "What it hasn't done," he pointed out with obvious pleasure (after all he can justly claim that he was at least partly responsible) "is to shout at the old friend from nearby - I mean, it hasn't produced a rather raucous, younger person standing beside, saying 'look how old and wrinkled you ar e', so to speak.... It has quite sensitively complemented the old building - whatever you may think of it." However, Charles doesn't - so to speak - "win them all." In Edinburgh on Aug. 13 the result of a competition for a new Museum of Scotland was announced. This is also an extension to an existing old building. The winner, out of a field of 371 entries from all over the world, is a small firm called Benson + Forsyth. A Scot is one-half of the partnership although the firm is London based. They beat into second place Ulrike Wilke with James Stirling, Michael Wilford and Associates. Stirling is a world-renowned architect with art galleries and museums his specialty. It just happens that Prince Charles was president of the National Museums of Scotland Patrons Organization. "Was" is the operative word because he resigned this position. His reasons were two-fold: that he had not been given sufficient say in the selection of architects and that the general public had not either. How the general public was to have been consulted, is not at all clear. They are now - an exhibition of the winning entries at the old museum is accompanied by a comments book. But who is going to take any notice of the comments in it?Charles's resignation was timed to coincide with the announcement o f the competition winner. And although he said he wasn't criticizing the winner's project by his action, nevertheless the effect of his resignation was to completely overshadow the announcement. The winners were exhaustively selected by a team of assessors, half of them professional architects and half laymen. The prince said that he was criticizing this selection method. The effect, however, was to also criticize the winners. The Earl of Perth, chairman of the Museum Patrons, observed: "It's always nice to have royalty on the notepaper," but added that this cause can stand on its own feet. Architect Stirling, however, has since publicly compared Charles's appeal to popular taste to Hitler's, and argued that the prince doesn't know good from bad in modern architecture. Ironically, the Benson + Forsyth building - which, say museum officials, will now undergo considerable changes as, for the first time, consultation takes place between the client and the architects - may well be to the prince's taste. It is full of historical references for a start. And the chief quiver in Charles's bow is "the past." He loved a covered shopping mall in the little Yorkshire town of Skipton that is pure Victorian pastiche and confection. And he wrote (in his own book "Vision of Britain,I believe that when a man loses contact with the past he loses his soul." But maybe - for all the undoubted healthiness of the arguments about architecture the prince has opened up by his brick-bats - maybe there are some 20th-century people who are not to be intimidated. And maybe these people are Scots.